Electrically conductive polymeric mixture, method of molding conductive articles using same, and electrically conductive articles formed therefrom

ABSTRACT

A thermally stable electrically conductive polymer mixture, for use in forming electrically conductive molded articles, includes at least two additives for contributing to the polymer mixture&#39;s electrical conductivity. At least one of the additives concentrates at or near the surface of a molded article for electrical surface conductivity, while at least one other additive concentrates at or near the core of a molded article to heighten electrical core conductivity to promote electrostatic discharge.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/832,239 filed Apr. 10, 2001, now U.S. Pat. No. 6,685,854, thecontents of which are incorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrically conductive polymercompositions, to methods of use thereof, and to electrically conductivearticles produced therefrom. More particularly, the present inventionrelates to a polymeric mixture containing at least two conductiveadditives, to provide both surface conductivity and internalconductivity, without significantly affecting the physical properties ofthe polymer. The present invention further relates to methods of formingmolded articles using the polymeric mixture, and to electricallyconductive molded articles formed therefrom.

2. Description of the Background Art

Selected electrically conductive polymers are known and used inindustrial settings, particularly in the manufacture of electroniccomponent parts. Some examples of electrically conductive polymercompositions are illustrated in U.S. Pat. Nos. 5,256,335; 5,281,363;5,378,403; 5,662,833; 5,958,303; 6,030,550; and 6,149,840.

U.S. Pat. No. 5,281,363 to Shacklette et al. is directed towardsdiscrete particles formed of an electrically conductive polyanilinecomposition, which may be between 0.05 microns and 100 microns in size.In the particles of the '363 patent to Shacklette et al, a first dopantpredominates at or near the surface of the particle, and a second dopantpredominates at or near the core of the particle. The disclosure ofShacklette et al. is not specifically directed to molded articles, or toconcentrations of electrically conductive additives in molded articles.The Shaklette patent offers surface and core conductivity within aparticle; however, this invention utilizes salts and/or acids as surfaceadditives which may provide a lower rate of conductivity than optimallypossible, and which may also afford a limited range of thermalstability.

U.S. Pat. No. 5,662,833 to Laasko et al. discloses electricallyconducting thermoset compositions, in which a thermoset matrix containsa polyaniline protonated with a protonic acid containing at least onehydroxyl group. In the Background section of Laasko, conductive plasticsare generally categorized as either filled conductive plastics orintrinsically conductive plastics. While this reference goes on to listseveral conductive particles which are usable as additives in filledconductive plastics, it does not teach or suggest using a combination ofthese additives to provide both surface conductivity and coreconductivity in a molded article.

While polymer compositions provide an electrically conductive medium,difficulty arises in creating molded articles having both efficientsurface conductivity and electrostatic discharge (ESD) capabilities. Toutilize both electrical surface conductivity and ESD efficiently,conducting agents are necessary additives to the polymer's composition.Selecting an additive that performs a dual function of surfaceconductivity and ESD is difficult, since efficient surface conductivityrequires that the additive concentrate on or near the surface of thepolymer, and efficient ESD requires that the additive concentrate aroundthe core of the polymer.

Adding a sufficient quantity of a single conducting agent to a polymer'scomposition to provide effective ESD, while also providing efficientsurface conductivity, often diminishes the physical properties of thepolymer. The problem of preserving the physical properties of a polymerwhile attempting to generate both efficient surface conductivity and ESDremains elusive.

As a result, a need still exists in the art of electrically conductivepolymer compositions for a material or mixture that provides bothsurface conductivity and ESD performance. In particular, there is a needfor a material that operates to conduct electrical current on thesurface area and provides electrostatic discharge without significantlydiminishing the physical properties of the polymer composition.

One example of a possible application of a conductive polymeric materialis in a plastic fuel filter housing. Contemporary fuel filters areformed using plastic housings in many instances today. Unfortunately,there is some risk that a fuel filter may build up static electricitywithin the plastic housing thereof.

In the event that a plastic fuel filter housing, having fuel storedtherein, was exposed to a spark caused by sudden electrostaticdischarge, the consequences could be hazardous if such a spark were toignite fuel in, on or near the fuel filter. Accordingly, it would bedesirable to form a plastic housing for a fuel filter in which thehousing could be electrically conductive, and could be connected toground in order to avoid the buildup of static electricity therein, andconsequent sudden electrostatic discharge.

SUMMARY OF THE INVENTION

The present invention has been developed to overcome the foregoinglimitations and disadvantages of known electrically conductive polymercompositions, and to generally fulfill a need in the art for a dualconductive polymer which provides both surface area electricalconductivity and electrostatic discharge, while maintaining the physicalproperties of the polymer composition.

The polymeric mixture according to the present invention conductselectricity primarily through the inclusion of electrically conductiveadditives, rather than by using inherently conductive polymers. Inpracticing the invention, two different additives are used, a firstadditive to provide surface conductivity and a second additive toprovide core conductivity.

An electronically conductive polymer composition, in accordance with thepresent invention, includes

a) a polymeric material;

b) a first electrically conductive additive selected from the groupconsisting of metallic fibers and metallic particles; and

c) a second electrically conductive additive selected from the groupconsisting of carbon fibers and carbon particles.

Such an electrically conductive polymer composition is advantageousbecause it provides both surface conductivity and electrostaticdischarge capability, without diminishing the physical properties of thepolymer composition.

Preferably, the polymeric material is a thermoplastic selected from thegroup consisting of polyamides, polyimides, polyesters, polyolefins,polysulfones, fluoropolymers, and mixtures thereof. Particularlypreferred polymeric materials are acetal and nylon 12.

Preferred additives are carbon particles and stainless steel fibers, andmost preferably, the mixture contains both carbon powder and stainlesssteel fibers together.

The present invention also relates to a method of making an electricallyconductive molded article, comprising the steps of:

a) injecting a polymer mixture into a mold cavity having formed within ahollow mold, said mold cavity having an outer periphery;

said polymer mixture comprising:

-   -   a polymeric material;    -   a first electrically conductive additive selected from the group        consisting of metallic fibers and metallic particles; and    -   a second electrically conductive additive selected from the        group consisting of carbon fibers and carbon particles;    -   whereby the first electrically conductive additive migrates away        from the outer periphery of the mold cavity, and the second        electrically conductive additive migrates toward the outer        periphery of the mold cavity;

b) curing the polymer in the hollow mold to form a molded article; and

c) ejecting the molded article from the mold.

The present invention also relates to an electrically conductive moldedarticle which is a product of the above-described process. Inparticular, one example of a useful molded article in accordance withthe invention is a fuel filter housing.

Accordingly, it is an object of the present invention to provide amixture which is suitable for forming electrically conductive moldedarticles able to conduct electric current within the surface areathereof, and consistently performing electrostatic discharge whilemaintaining the structural integrity of the polymer composition.

For a more complete understanding of the present invention, the readeris referred to the following detailed description section, which shouldbe read in conjunction with the accompanying drawings. Throughout thefollowing detailed description and in the drawings, like numbers referto like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a sequence of steps in forming amolded article according to a method of the present invention;

FIG. 2 is a cross-sectional view of a fuel filter; and

FIG. 3 is a simplified cross section, partially cut away, of part of aside wall of the fuel filter housing of FIG. 2, taken along the line3—3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first embodiment thereof, the present invention provides acomposition which is a mixture of a polymer and at least two differentelectrically conductive additives. A mixture in accordance with thepresent invention, generally, includes:

a) a pulverized polymeric material;

b) a first electrically conductive additive selected from the groupconsisting of metallic fibers and metallic particles; and

c) a second electrically conductive additive selected from the groupconsisting of carbon fibers and carbon particles.

Preferably, the pulverized polymeric material is a thermoplastic.Suitable thermoplastic materials usable in the practice of the presentinvention include polyamides, polyimides, polyesters, polyolefins,polysulfones, fluoropolymers, and mixtures thereof.

One particularly preferred thermoplastic is polyacetal resin polymer,also called polyoxymethylene, which is a product of the polymerizationof a formaldehyde-containing starting material. This material is oftenreferred to by the abbreviated name “acetal” or “POM”. One suitable typeof acetal resin polymer, which may be used in the practice of thepresent invention, is that sold commercially by DuPont under thetrademark “DELRIN”. Another suitable thermoplastic is Nylon 12.

Most preferably, the mixture according to the invention includes bothcarbon particles and stainless steel fibers.

The present invention also relates to a method of making an electricallyconductive molded article, comprising the steps of:

a) injecting a polymer mixture into a mold cavity having formed within ahollow mold, said mold cavity having an outer periphery;

said polymer mixture comprising:

-   -   a polymeric material;    -   a first electrically conductive additive selected from the group        consisting of metallic fibers and metallic particles; and    -   a second electrically conductive additive selected from the        group consisting of carbon fibers and carbon particles;    -   whereby the first electrically conductive additive migrates away        from the outer periphery of the mold cavity, and the second        electrically conductive additive migrates toward the outer        periphery of the mold cavity;

b) curing the polymer in the hollow mold to form a molded article; and

c) ejecting the molded article from the mold.

The above-described method may also include preparatory steps of mixingthe additives into the pulverized polymer, and melting the polymer,where a thermoplastic is used, to form the flowable mixture which isthen injected into the mold.

Referring now to FIG. 1, a series of steps in one particularly preferredmethod according to the invention is shown schematically.

An initial step of mixing the additives into the dry pulverized polymeris shown at 20. As previously noted, the most preferred additives arecarbon particles and stainless steel fibers.

The next step, shown at 22 in FIG. 1, is melting the thermoplasticpolymer to form a flowable mixture.

Then, in an injection step 24, the flowable mixture is injected into amold cavity using conventional injection molding equipment. During andimmediately subsequent to the injection step 24, the heated mixtureexhibits a natural tendency for the carbon particles to migrate towardsthe outer periphery of the mold, while the steel fibers remain at ornear the core.

After the injection step, the mold is cooled in a cooling step 26,allowing the polymer to set and form a molded article 10 (FIG. 2). Inthe example shown in FIG. 2, the molded article is part of a housing fora fuel filter 12.

The finished article is then ejected from the mold in an ejection step28.

The present invention also relates to an electrically conductive moldedarticle which is a product of the above-described process. Inparticular, one example of a useful molded article 10 in accordance withthe invention is a fuel filter housing.

Referring now to FIG. 3, a portion of a side wall 14 of the article 10is shown. In the cross-sectional view of FIG. 3, it can be clearly seenthat the carbon particles 16 tend to concentrate at or near the surface15 of the article 10, which gives the article good surface conductivity.In contrast, the stainless steel fibers 18 tend to remain at or near thecenter or core 17 of the article 10, which advantageously also providesinternal conductivity which promotes electrostatic discharge (ESD).

Although the present invention has been described herein with respect toa preferred embodiment thereof the foregoing description is intended tobe illustrative, and not restrictive. Those skilled in the art willrealize that many modifications of the preferred embodiment could bemade which would be operable. All such modifications which are withinthe scope of the claims are intended to be within the scope and spiritof the present invention.

1. A method of making an electrically conductive fuel filter housing,comprising the steps of: a) injecting a flowable mixture into a moldcavity having formed within a hollow mold configured to shape a fuelfilter housing structure, said mold cavity having an outer periphery;said flowable mixture comprising: a polymeric material; a firstelectrically conductive additive selected from the group consisting ofmetallic fibers and metallic particles; and a second electricallyconductive additive selected from the group consisting of carbon fibersand carbon particles; whereby the first electrically conductive additivemigrates away from the outer periphery of the mold cavity, and thesecond electrically conductive additive migrates toward the outerperiphery of the mold cavity; b) curing the polymer in the hollow moldto form a molded fuel filter housing structure; and c) ejecting themolded fuel filter housing structure from the mold.
 2. The method ofclaim 1, wherein the polymeric material is a thermoplastic selected fromthe group consisting of polyamides, polyimides, polyesters, polyolefins,polysulfones, fluoropolymers, and mixtures thereof.
 3. The method ofclaim 2, wherein the polymeric material is selected from the groupconsisting of acetal and nylons.
 4. The method of claim 1, wherein thefirst electrically conductive additive comprises stainless steel fibers.5. The method of claim 4, wherein the second electrically conductiveadditive comprises carbon particles.
 6. The method of claim 3, whereinthe polymeric material comprises Nylon 12.